The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with black blood magnetic resonance angiography and may be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with other types of angiography and other types of magnetic resonance imaging.
Measurement of blood flow, in vivo, is important for the functional assessment of the circulatory system. Angiography has become a standard technique for making such functional assessments. Magnetic resonance angiography (MRA) provides detailed angiographic images of the body in a non-invasive manner, without the use of contrast agents or dyes.
Traditionally, MRA methods can be divided into xe2x80x9cwhite bloodxe2x80x9d and xe2x80x9cblack bloodxe2x80x9d techniques. In white blood angiographs or time of flight (TOF) angiographs, magnetic resonance signal from flowing blood is optimized, while signal from stationary blood or tissue is suppressed. This method has been problematic for a number of reasons. First, it is difficult to generate accurate images of the vascular system because the excited blood is constantly moving out of the imaging region. Also, blood vessels often appear more narrow because signal from the slow-flowing blood at the edges of the vessels is difficult to detect.
In contrast, black blood angiography methods utilize a flow-related signal void. The magnetic resonance signals from flowing blood are suppressed, while the signals stationary blood and tissue are optimized. In other words, flowing blood is made to appear dark or black on the magnetic resonance image due to an absence or minimum of resonance signal emanating from the blood. The black blood method is typically preferable to the white blood method because it is easier to make flowing blood appear dark for the aforementioned reasons. In addition, blood vessels on a black blood angiograph appear larger because the slow-moving blood at the edges is clearly imaged. Also, the black blood MRA provides more detailed depiction of small vessels where blood flow is slower.
In black blood MRA, flow-related signal void can be generated by using spoiling gradients, pre-saturation RF pulses, or defocused flowing spins. The first two means are mostly used in field echo (FE) style sequences while the latter one is typically used in spin echo (SE) style sequences, such as fast spin echo (FSE) sequences.
In the past, proton density weighted (PDW) FSE sequences have been used to acquire images. For a sixteen echo FSE sequence, the first echo is oriented near the center of k-space, the second echo is located in the adjacent segment, and so on. In such an arrangement of k-space data, PDW images are acquired. These images result in good background for images. However, slow-flowing dipoles are refocused by the subsequent 180xc2x0 pulses contributing signal and resulting in xe2x80x9cfillingxe2x80x9d, i.e., black blood in the center of arteries and veins and white or gray blood along the blood vessel walls, in capillaries, and in areas with slower moving blood. The filling effect leads to falsified vessel definition. While this problem may be resolved by using pre-saturation RF pulses, this comes at the cost of increasing SAR which is very critical on a high field system (xe2x89xa71.5 T).
The present invention contemplates a new method and apparatus which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of generating a black blood magnetic resonance angiograph is provided. Dipoles within a selected imaging region are excited to produce magnetic resonance signals. A train of magnetic resonance echoes is induced after the excitation. Early echoes are more heavily proton density weighted and later echoes are more heavily T2 weighted. The train of magnetic resonance echoes is phase and frequency encoded. The echoes are received and demodulated into a series of data lines. The data lines are sorted between data lines from more heavily proton density weighted echoes and data lines from more heavily T2 weighted echoes. The more heavily proton density weighted data lines are reconstructed into a proton density weighted image representation; and the more heavily T2 weighted data lines are reconstructed into a T2 weighted image representation. The proton density weighted and T2 weighted image representations are combined to generate a black blood angiographic image representation.
In accordance with another aspect of the present invention, a magnetic resonance imaging system includes a magnet for generating a temporally constant magnetic field through an examination region. A radio frequency transmitter excites and inverts magnetic dipoles in the examination region. Gradient magnetic field coils and a gradient magnetic field controller generate at least phase and read magnetic field gradient pulses across the examination region. A receiver receives and demodulates the magnetic resonance echoes to produce a series of data lines. A sorter sorts the data lines between proton density weighted data lines and T2 data lines. An early echo volume memory stores the proton density weighted data lines and a late echo volume memory stores the T2 weighted data lines. An image processor reconstructs the proton density weighted image lines into a proton density weighted image representation and the T2 weighted data lines into a T2 weighted image representation. A combination processor combines the proton density weighted and the T2 weighted image representations.
One advantage of the present invention is that it is more scan time efficient.
Another advantage of the present invention is that a proton density weighted image, a T2 weighted image, and a black blood angiogram are acquired from a single scan.
Another advantage of the present invention is that it leads to more accurate vascular morphology.
Another advantage of the present invention is that it eliminates mis-registration error between proton density weighted images and T2 weighted images.
Yet another advantage of the present invention is that it reduces the SAR level.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.